1. Field of the Invention
The present invention relates to a driving circuit for a coil, particularly for driving a coil in an instrument such as a camera whose shutter blades are driven with the electro-magnetic force generated by interaction between a current flowing through the coil and a magnetic field in which the coil is positioned.
2. Description of the Prior Art
In cameras which achieve an electro-magnetic driving force by virtue of current flowing through a coil placed in a magnetic field and drive shutter blades with the electro-magnetic driving force (hereinafter referred to as cameras employing an electro-magnetic shutter driving force), the shutter blades are directly driven by the electro-magnetic force. Therefore, unless a constant electro-magnetic driving force can be obtained, a correct control of shutter driving cannot be obtained and a correct exposure control cannot be expected.
That is, since the running speed of the shutter blades themselves change depending on the electro-magnetic driving force in an above-mentioned type of camera, it becomes necessary always to obtain a constant electro-magnetic driving force.
Because of this necessity, it is particularly desirable that the coil current flowing through the coil be regulated to a stable constant value over temperature and that the voltage variation of a power source in such type of camera also be constant.
To meet this requirement, it would be possible to use a known type of constant current driving circuit (shown in FIG. 1) as a coil driver in such a camera. Here, a reference voltage V.sub.ref which is derived, for example, from a well known band-gap reference voltage circuit and is stable with temperature is applied to a non-inverting input terminal of an operational amplifier 101. A voltage across a resistor 103 (whose value is supposed to be stable over temperature and is represented by R) which represents a current flowing through a coil 104 is applied to an inverting input terminal of said operational amplifier 101, form a negative feedback circuit. The voltage across the resistor 103 always is made equal to V.sub.ref and current represented by a formula, (V.sub.ref /R), is always made to flow through the coil 104, so that the coil current will not be affected by a variation in a power source voltage and a temperature variation.
However, when such method is applied to a coil driving circuit in a camera using an electro-magnetic driving force for shutter, the following shortcomings will occur.
Since a relatively great power is needed as an electro-magnetic force in the above-mentioned type of camera, the product of the current flowing through the coil and the number of turns of the coil needs to be made larger, and as a result, a value of
(coil resistance).times.(coil current)=(voltage across the coil)
cannot be made small.
Also, there will be a certain restriction in the number of turns of coil from various aspects such as a weight and size of the coil, and as a result the coil current itself needs to have a greater value (several hundreds of mA) than that of an ordinary solenoid for a shutter. Thus, a relatively low voltage such from a battery has inevitably to be used for V.sub.BAT in FIG. 1 without boosting since a power source needs to be made small in application to a small size apparatus such as a camera, even if a relatively high power source voltage from a booster circuit such as DC/DC converter, can be used for a control circuit with small power consumption. With this restriction, it is not desirable from a practical point of view that a minimum operation voltage be raised by the voltage across the resistor 103.
Such apparent rising of the minimum operation voltage may be accepted to some extent when the values of the reference voltage V.sub.ref and the resistor 103 are reduced. However, when a coil current is adjusted, the reduced V.sub.ref or the resistor value R requires a further fine adjustment. This involves some difficulty is involved.
On the other hand, according to another well known driving circuit, it is possible that a reference voltage V.sub.ref such as shown in FIG. 1 is applied to a non-inverting input terminal of an operational amplifier 201, as shown in FIG. 2, and a voltage obtained by dividing a voltage across a coil 205 with resistors 203 and 204 (the values of which are represented by R.sub.1, R.sub.2, respectively) is fed back to an inverting-input of the amplifier 201.
In FIG. 2, since a negative feedback loop is formed such that the manner that a voltage across the resistor 203 becomes equal to the V.sub.ref, the voltage across the coil 205 is always controlled to a value represented by EQU V.sub.ref .times.(R.sub.1 +R.sub.2 /R.sub.1),
and,
thus, the current flowing through the coil will be constant at a constant temperature with power source variation. Hence, it becomes possible to compensate for the temperature coefficient of a coil resistance with temperature by providing an appropriate temperature coefficient for each of the resistors 203, 204, thus restraining a change in the coil current over temperature.
However, supposed that the temperature coefficient of the coil resistance is equivalent to that of a copper wire, namely about +3,900 ppm/.degree.C., while temperature coefficients of an ordinary carbon film resistor, and a metal film resistor are .+-. a few or several hundreds of ppm/.degree.C. Therefore, a special material for resistor will be needed to effect temperature compensation of a coil current with the schematic shown in FIG. 2. Thus the compensation will be difficult to apply for an industrial mass production, because an instability in performance, a nonlinearity and a disadvantage in a cost constitute unavoidable obstacle as far as a current level of related industries is concerned.
Further, when the resistor 203 or the resistor 204 needs to be a variable resistor for adjustment of a coil current in the above method, the difficulty becomes greater.